I canoed down the Whanganui over Christmas a couple of years ago. It's one of New Zealand's 'Great Walks' with huts to stay in along the way.

We were told that the Māori considered the river to be a person and as a sign of respect we should not wash ourselves in it. Aside from that, it did not effect our trip.

If anyone is interested, there's more information here - https://www.doc.govt.nz/parks-and-recreation/places-to-go/ma...